it depends on the cpu. going 2.0v on a thouroughbred or a duron is usually too high unless you have supercooling (however, unlike the northwoods, the tbreds do not seem to die from high voltages as i have not read a single thread as of yet on any forum saying that a tbred has kicked in because of high voltages).
if you have a palomino or thunderbird, 2.3v will not kill it , but that is only if you are using very good cooling. we are talking water w/ 225w pelt or a good phase change unit (prometia or homemade).... or other crazy cooling
I wouldn't recommend running more than 2v on water or anything higher than 2.1 on super cooling, although the tbreds seem to hang in there w/ the high vcores, its still a 0.13µm proc which is much more sensitive to voltage.
I'd have to agree I wouldn't go over 2V...I'm luck my TB still runs.
I'm using 2.2V on air cooling with the volcano 9, and get 40C idle to 53C load. I originally could run my system at 1.9V @1440 but now after overheating on a generic heatsink I have to go very high voltage to get this same speed, but I plan on getting a XP chip soon, so I'm not worried.
If you do, Go with WaterCooling, and besure to use a high water pump so it doesn't overheat the chip.